1. Field of the Invention
The present invention relates to a vessel pulse wave measurement system, and more particularly, to a vessel pulse wave measurement system that performs vessel pulse wave measurement by obtaining a pulsation waveform (hereinafter, referred to as a pulse wave) of a blood vessel using a light emitting element and a light receiving element.
2. Description of the Related Art
For techniques for evaluating properties of a material, a method using an oscillation has been known. Patent Document 1 discloses a method of converting phase change to frequency change, taking into account that in the difference in the properties of the material, a change in the phase of oscillation is greater than a change in the frequency of oscillation but the accuracy of phase measurement techniques is not always high. An apparatus using this method is configured to include an oscillator, an oscillation detection sensor, an amplifier, a phase shift circuit, and amount-of-frequency-change detection means. The oscillator oscillates and radiates ultrasound into the material, and the oscillation detection sensor detects a reflected wave from the material. The amplifier has an input terminal connected to a signal output terminal of the oscillation detection sensor, and the phase shift circuit is provided between an output terminal of the amplifier and an input terminal of the oscillator, and changes the frequency to shift the phase difference to zero when a phase difference occurs between an input waveform to the oscillator and an output waveform from the oscillation detection sensor. The amount-of-frequency-change detection means detects the amount of frequency change for shifting the phase difference to zero.
In the apparatus of Patent Document 1, specifically, a hardness measuring apparatus using a frequency deviation detection circuit is configured to include a contact element, an oscillator, a self-oscillation circuit, and a gain change correction circuit in order to accurately measure hardness information in a wide range from a soft object to be measured to a hard object to be measured. The self-oscillation circuit feeds back oscillation information of the oscillator to bring about a resonance state. The gain change correction circuit is provided in the self-oscillation circuit. The gain change correction circuit has a center frequency different from that of the self-oscillation circuit, and increases the gain in response to a change in the frequency.
In the above-described apparatus, the amount-of-frequency-change detection means shifts a phase difference caused by a difference in hardness to zero, and converts the resulting shifted phase difference to an amount of frequency change. The conversion uses a reference transfer function representing the relationship between the amplitude gain and phase of a reflected wave with respect to frequency, where the reference transfer function is obtained in advance. In addition, although ultrasonic oscillation is used as oscillation, instead of this, oscillation of an electrical signal in an electric circuit can be used. For example, a light emitting element is driven by a drive signal to radiate light, and a light receiving element detects the light and feeds back a detected signal as a drive signal of the light emitting element. Then a feedback loop is formed, and oscillation of an electrical signal flowing through the feedback loop can be used.
Namely, there is a signal delay between a drive signal of the light emitting element and an optical signal to be radiated, which is caused by the structure of the light emitting element, and likewise, there is also a signal delay between an optical signal entering the light receiving element and a detected signal outputted from the light receiving element, which is caused by the structure of the light receiving element. Therefore, if a feedback loop is formed by combining the light emitting element and the light receiving element, then self-oscillation occurs so as to make a phase difference, which is a delay therebetween zero. By providing the phase shift circuit disclosed in Patent Document 1 in the feedback loop, the phase difference can be converted to a frequency difference.
Then, the light from the light emitting element is allowed to radiate an object to be evaluated, and the light receiving element receives light reflected from the object. In this case, a feedback loop is formed, then the frequency of the self-oscillation circuit depends on a delay caused by the configuration of the light receiving element and the light emitting element and a delay caused by the properties of the material to be evaluated. Therefore, by providing a phase-shift circuit in the feedback loop and converting a phase difference by each frequency and observing the frequency difference, the properties of the material can be measured in a non-contact manner or non-invasive manner.
For example, Patent Document 2 describes a blood pressure measuring apparatus that includes a sensor unit, and a self-oscillation circuit. The sensor unit transmits infrared light into the body and receives a reflected wave in the body, and the self-oscillation circuit performs self-oscillation by feeding back an electrical signal based on the received reflected wave to a wave transmitting unit. The self-oscillation circuit includes a gain change correction circuit that changes gain in response to frequency change, and adjusts a phase difference between an input phase and an output phase to zero to promote feedback oscillation. A blood pressure can be calculated based on an oscillation frequency of the self-oscillation circuit obtained in the above-described manner.
In the apparatus of Patent Document 2, in order to perform measurement of blood pressure with high accuracy and to reduce burdens on a person to be measured, a wave transmitting unit converts an electrical signal and transmits an electromagnetic wave or ultrasonic wave such as infrared light into the body. Then a wave receiving unit receives a reflected wave in the body, and converts the reflected wave to an electrical signal. The frequency of the self-oscillation circuit measured by a frequency measuring unit is converted to a blood pressure value based on a correlation parameter invoked by a blood pressure computing unit, and a display unit sequentially provides display of the blood pressure value or a blood pressure waveform.
As mentioned above, according to the technique of the phase shift method, a pulsation waveform of a blood vessel can be accurately obtained using the light emitting element and the light receiving element. However, a living body, which is a target for measuring pulsation of a blood vessel, such as a person to be measured, does not always maintain a stable state during measurement. The living body may change his/her posture such as moving his/her arm having the light emitting element and the light receiving element attached thereto. In addition, if the attachment state of the light emitting element and the light receiving element is incomplete, then the attachment state may change during measurement.
Accordingly, a pulsation waveform may change gradually during measurement and may, for example, go out of a measurement range and a computing range. If the pulsation waveform thus shifts from the measurement range, then accurate vessel pulse wave measurement cannot be performed. In order to provide a vessel pulse wave measurement system capable of performing more accurate measurement to solve this problem, the inventors of this application proposed the following vessel pulse wave measurement system in Patent Document 3.
The vessel pulse wave measurement system of Patent Document 3 is characterized in that the system includes an optical probe, a pulsation waveform output unit, and an arithmetic processing unit. The optical probe is attached to a part suitable for obtaining pulsation of a blood vessel of a person to be measured. The pulsation waveform output unit is connected to the optical probe through an optical probe circuit, and outputs a pulsation waveform as a temporal change in frequency, using a phase shift method. A floating median setting processing module of the arithmetic processing unit has a function of amplifying the maximum amplitude value of periodic frequency data such that the maximum amplitude value has a predetermined ratio with respect to a computing range, and setting a median thereof as a median of the computing range in a floating manner, regardless of an absolute value thereof.
Further, Patent Document 4 proposes an abnormal respiration detection apparatus including abnormal respiration determination means for obtaining the number of pulses and pulse amplitude based on a pulse wave signal indicating the state of a pulse wave, and making a determination of abnormal respiration based on the number of pulses and the pulse amplitude. The apparatus is characterized by, for example, detecting abnormal respiration based on the ratio between pulse wave amplitude and the number of pulses per unit time, or detecting abnormal respiration based on a change in the number of respirations, a change in the number of pulses, and a change in oxygen saturation concentration in blood.
Prior art documents which related to the present invention are as follows.    Patent Document 1: Japanese patent laid-open publication No. JP 9-145691 A;    Patent Document 2: Japanese patent laid-open publication No. JP 2001-187032 A;    Patent Document 3: International publication No. WO 2010/089893;    Patent Document 4: Japanese patent laid-open publication No. JP 2004-121668 A;    Patent Document 5: Japanese patent laid-open publication No. JP 6-169892 A;    Patent Document 6: Japanese patent laid-open publication No. JP 2005-021477 A; and    Non-Patent Document 1: Motoaki Sugawara et al., “Hemorheology and Blood Flow”, Corona Publishing Co., Ltd., pages 120-121, Apr. 25, 2003, together with Partial translation.
However, the apparatuses according to the prior art disclosed in the above-described Patent Documents 1 to 3 have such a problem that the apparatuses do not work almost at all on the measurement scene because of frequent occurrence of the case in which the apparatuses cannot obtain pulsation waveform data due to that the operation of obtaining pulsation of a blood vessel often becomes an unstable state,
(a) in addition to a change in the attachment state of the light emitting element and the light receiving element,
(b) according to the attachment part; for example, whether to attach the light emitting element and the light receiving element to the radial artery portion at the wrist or to a fingertip,
(c) in addition, for example, according to the thickness of the skin of a thin person to be measured or a fat person to be measured, even if the light emitting element and the light receiving element are attached to the same part of the radial artery portion, and
(d) further, according to the type of optical probe; for example, whether to use a reflective type optical probe using reflected light from a blood vessel or use a transmission type optical probe using transmitted light transmitted through a blood vessel.
In addition, the abnormal respiration detection apparatus according to the prior art disclosed in the Patent Document 4 has such a problem that the accuracy of detection of abnormal respiration is still low, in addition to the above-described problem.